The invention relates to a process for preparing crystals of Hg.sub.1-x Cd.sub.x Te of high purity according to a so-called THM (Travelling Heater Method) technique, whereby ingots of compounds entering in the composition of the crystal to be obtained are caused to pass through a zone of solvent.
The ternary compound Hg.sub.1-x Cd.sub.x Te is a semiconductor of which the forbidden band width varies with the proportion x of cadmium. Depending on the proportion x chosen, photodetectors may be obtained whose spectral sensitivity is located in different domains of the infrared (about 1.5 .mu.m for x.perspectiveto.0.7, about 10 .mu.m for x.perspectiveto.0.20).
The article by R. Triboulet, "CdTe and CdTe:Hg alloys crystal growth using stoichiometric and offstoichiometric zone passing techniques" which appeared in the Revue de Physique Appliquee, Vol. 12, February 1977, page 123, describes the preparation of crystals of Hg.sub.1-x Cd.sub.x Te, with x.perspectiveto.0.9 from an ingot of HgTe and an ingot of CdTe, with the aid of tellurium as solvent. The two source ingots move at a very slow speed with respect to the appropriately heated solvent zone. The part of the source ingots which penetrates in the solvent zone dissolves and a single crystal of Hg.sub.1-x Cd.sub.x Te is formed on leaving this zone, the proportion x of cadmium being a function of the ratio between the sections of the source ingots.
On reading the above-mentioned article, it is observed that the composition of the ingot of Hg.sub.1-x Cd.sub.x Te obtained is not constant over the whole of its length. In fact, it is observed that the head of the ingot presents a greater proportion of cadmium than the desired value x, and that consequently the desired proportion x is obtained only over part of the ingot, representing about 60 to 70% of its length.
The problem which resulted in the instant invention and which the inventors, MM. TRIBOULET, DIDIER, DURAND and ROYER, (the two first mentioned inventors working with the CNRS Bellevue Solid Stages Physics Laboratory), wanted to solve consequently consists in eliminating this undesirable variation in the composition at the head of the ingot, so that virtually the whole ingot presents the desired composition and therefore the desired spectral sensitivity.
It has been discovered that this variation in the composition was attributable to the fact that the solvent, composed solely of tellurium at the beginning of the process, became initially rich in CdTe and HgTe in a ratio equal to that of the sections of the source ingots, corresponding to a proportion x.sub.L of CdTe, but that the ingot which is then formed on leaving the solvent zone presents a proportion x.sub.S of cadmium, determined by the phase chart, which is greater than x.sub.L.
Due to this difference between x.sub.L and x.sub.S, the taking of CdTe from the solvent zone is greater than the addition of CdTe and progressively, the solvent zone becomes lean in cadmium and, inversely, becomes rich in mercury. This is accompanied by a concomitant reduction of the difference between x.sub.L and x.sub.S and a state of thermodynamic balance is finally reached in which the ingot formed presents a proportion x.sub.S equal to the desired proportion x.sub.L corresponding to the ratio of the sections of the source ingots, and the solvent zone contains determined amounts of CdTe and HgTe, in addition to the tellurium.